Growth factors are substances, such as poly-peptide hormones, which affect the growth of defined populations of animal cells in vivo or in vitro, but which are not nutrient substances. Proteins involved in the growth and differentiation of tissues may promote or inhibit growth, and promote or inhibit differentiation, and thus the general term "growth factor" includes cytokines and trophic factors.
Growth factors typically are polypeptides ranging in molecular weights from 5000 to 50,000 daltons. Based on structural similarities, growth factors are categorized into families which include: insulin-like growth factors (IGFs), platelet-derived growth factors (PDGFs), fibroblast growth factors (FGFs), epidermal growth factors (EGFs), nerve growth factors (NGFs), and transforming growth factors type-beta (TGF-.beta.s).
Transforming growth factor-.beta.s were originally named for their ability to transform normal fibroblasts to cells capable of anchorage-independent growth. However, despite the name, TGF-.beta.s are multifunctional growth factors that are required for the normal development, growth, and differentiation of various epithelial, endothelial, and mesenchymal cells. As with other cytokines, the specific effect of TGF-.beta.s depend on the particular cell type and its surrounding environment.
The effects of TGF-.beta.s on cells are generally classified as proliferative and non-proliferative. As originally established with the first experiments on fibroblasts, TGF-.beta.s are bona fide growth factors. Two important cell types in which proliferation is enhanced by TGF-.beta. are osteoblasts and Schwann cells of the peripheral nervous system. However, in many cells, TGF-.beta.s are potent inhibitors of cell proliferation. This negative growth control may be the regulatory mechanism that checks regeneration of certain tissues and may play a role in the initiation of carcinogenesis.
The most important non-proliferative function of TGF-.beta.s are in enhancing the formation of extracellular matrices. Although this is achieved primarily through the increased transcription of both collagen and fibronectin, the inhibition of the proteases from degrading the matrix also contributes to its stability. Degradation of the extracellular matrix is inhibited by the decrease in the secretion of the proteases themselves and the simultaneous increase in the levels of protease inhibitors. The marked and generalized effect of TGF-.beta. on extracellular matrices is likely to play a major role in tissue repair processes and the pathogenesis of certain fibrotic diseases.
DNA encoding several different receptors for TGF-.beta. has recently been described by Lin et al., PCT application WO93/09228, published May 13, 1993. The availability of the TGF-.beta. receptors will facilitate further assessments of TGF-.beta. functions.
Many members of the TGF-.beta. super family have been characterized. For example, Basler et al. have graphically represented the sequence relationship between members of the TGF-.beta. superfamily. Cell, 73, pp. 687-702 (1993). Massague, Annu. Rev. Cell Biol., 6, pp. 597-641 (1990) also reviews the transforming growth factor-.beta. family, including a discussion of the mechanisms of TGF-.beta. actions. An NMR characterization of the secondary structure of TGF-.beta.1 has been reported, and a refined 3-dimensional crystal structure of TGF-.beta.2 described, by Daopin et al., Proteins, 17, pp. 176-192 (1993). The monomer of TGF-.beta.2 adopts a fold that resembles a slightly curled left hand with two anti-parallel .beta.-sheets forming four fingers of the hand. These four finger regions together with conserved disulfides define the fold for the TGF-.beta. superfamily.
Also among TGF-.beta. members are the bone morphogenetic proteins (BMP). The BMPs have been indicated as useful in wound healing, tissue repair, and to induce cartilage and/or bone growth. For example, PCT Application 9309229, inventors Israel and Wolfman, published May 13, 1993, describes uses of proteins with bone stimulating activity such as bone fracture healing and possibly the treatment of periodontal disease and other tooth repair processes. A recent special article by C&EN, Hubbell and Langer, pp. 42-54 (Mar. 13, 1995) reports that a BMP has been incorporated into polymer particles so that as the polymer degrades, the protein is slowly released to surrounding tissues, where it stimulates the migration of cells into the porous matrix and, ultimately, the synthesis of new bone. The article also notes that bone has been produced by slowly releasing TGF-.beta..
Because of the wide applicability of TGF-.beta.s in clinical therapies, they have been the focus of much research. Although much of the research involved in vitro uses, recent in vivo studies have confirmed some of the more promising in vitro effects. As a consequence, some of the possible clinical uses for TGF-.beta.s include the stimulation of angiogenesis, the formation of granulation tissue associated with wound healing, and the formation of bone and cartilage.
Nucleic acid encoding TGF-.beta. and a variety of uses for TGF-.beta. are described in U.S. Pat. No. 5,284,763, issued Feb. 8, 1994, inventors Derynk and Goeddel. U.S. Pat. No. 5,258,029, issued Nov. 2, 1993, inventors Chu et al. describe preparations of stress-bearing prothesis with bony ingrowth occurring after implantation, which prothesis includes TGF-.beta. carried by a collagen composition or a ceramic. U.S. Pat. No. 5,368,858, issued Nov. 29, 1994, inventor Hunziker describes preparations of biodegradable matrices including TGF-.beta.s as proliferation agents, chemotactic agents, and transforming factors.
U.S. Pat. No. 5,055,447, inventors Palladino et al., issued Oct. 8, 1991, describes methods and compositions for the treatment or prophylaxis of Septic shock caused by bacteremic infection. Thus, for example, this patent teaches a therapeutic method for a patient suffering from or at risk of septic shock by administering transforming growth factor-.beta.. Recently, the concept of "sepsis" has been viewed more broadly as an inflammatory condition, and a group of researchers have suggested the designation "systemic inflammatory response syndrome" to describe both "sepsis" (infection by the presence of bacteria in the blood stream) as well as other (non-septic) inflammatory conditions. Chest, 101, pp. 1644-1655 (1992).
Thus, growth factors are useful in a number of therapeutic, clinical, research, diagnostic, and drug design applications. However, as previously mentioned, growth factors are typically large. The natural members of the transforming growth factor-.beta. family range upwards of 25 KDa molecular weight. Clinical uses of growth factors, including TGF-.beta.s, may be limited because of their size, such as due to causing immune responses. For example, human TGF-.beta.1 is a 25,000 dalton homodimeric protein. In addition to possible adverse immunological responses, large proteins are not often the best candidates for drugs because of the difficulties in administration and delivery.
Consequently, small peptide mimics of natural growth factors which would avoid most of these problems would be desirable for applications including those to which TGF-.beta. has been put or suggested. It would be advantageous to have small peptides mimicking the biological activity of the large, natural members since small peptides on a mole per mole basis would require much smaller net amounts for administration, and topical applications would be more feasible. Also, quite small peptides would tend to have little or no adverse immunological responses, and could be synthesized easily using simple peptide chemistry procedures.